Polydipsia is the first symptom occurs in Diabetes mellitus.it occurs associated with polyuria.
Lets see the association and Pathophysiology of these symptoms:
As you know in DM there is an increase in blood glucose level, now when that level exceed the kidney's threshold (renal threshold is the maximum amount of glucose which the kidney can filter and reabsorb, and its about 180 mg/dl), when the blood glucose level exceeds this limit, glucose will be "spilled" in the urine, and it wont be reabsorbed by the proximal renal tubule, that will lead to an increased osmotic pressure, and the osmotic pressure will attract more water, and by that we see the first symptom, which is frequent urination (polyuria), and because of all of the fluid loss through the urine, the blood volume will drop and dehydration may develop and this will trigger the thirst centers in the body, and voila polydipsia occur.
